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In their enveloping blueness, dotted with colorful fish and coral reefs, the undersea scenes are, in fact, quite meditative.

Critical infrastructure such as undersea cables would "continue to be a target", the document added.

The nation needs to do more to protect its critical national infrastructure, including undersea pipes and cables.

He cited recent attacks on undersea cables in the Baltic Sea, cyber attacks on European public transport, and unidentified drones spotted over German power stations and other infrastructure.

A mysterious and highly active undersea volcano off the Pacific Coast could erupt by the end of this year, scientists say.